Expression of murine NKG2D ligands on tumor cells has been shown to be effective in activating NK-mediated tumor elimination experimentally (Cerwenka et al., “Retinoic Acid Early Inducible Genes Define A Ligand Family for the Activating NKG2D Receptor in Mice,”Immunity 12: 721-7 (2000); Diefenbach et al., “Rae1 and H60 Ligands of the NKG2D Receptor Stimulate Tumour Immunity,” Nature 413: 165-71 (2001); Cerwenka et al., “Ectopic Expression of Retinoic Acid Early Inducible-1 Gene (RAE-1) Permits Natural Killer Cell-mediated Rejection of a MHC Class I-bearing Tumor In Vivo,” Proc Natl Acad Sci USA 98: 11521-6 (2001); Diefenbach et al., “A Novel Ligand for the NKG2D Receptor Activates NK Cells and Macrophages and Induces Tumor Immunity,” Eur J Immunol 33: 381-91 (2003)). In murine systems, identified NKG2D ligands include the retinoic acid early inducible family of proteins RAE-1 (Cerwenka et al., “Retinoic Acid Early Inducible Genes Define A Ligand Family for the Activating NKG2D Receptor in Mice,” Immunity 12: 721-7 (2000); Diefenbach et al., “Rae1 and H60 Ligands of the NKG2D Receptor Stimulate Tumour Immunity,” Nature 413: 165-71 (2001)), the minor histocompatibility antigen H60 (Cerwenka et al., “Retinoic Acid Early Inducible Genes Define A Ligand Family for the Activating NKG2D Receptor in Mice,” Immunity 12: 721-7 (2000); Diefenbach et al., “Rae1 and H60 Ligands of the NKG2D Receptor Stimulate Tumour Immunity,” Nature 413: 165-71 (2001)), and the murine ULBP-like transcript 1 (Diefenbach et al., “A Novel Ligand for the NKG2D Receptor Activates NK Cells and Macrophages and Induces Tumor Immunity,” Eur J Immunol 33: 381-91 (2003); Carayannopoulos et al., “Cutting Edge: Murine UL16-binding Protein-like Transcript 1: A Newly Described Transcript Encoding a High-affinity Ligand for Murine NKG2D,” J Immunol 169: 4079-83 (2002)). Cells expressing these molecules are sensitive to the cytotoxicity of mouse NK cells. Ectopic expression of RAE-1 and H-60 results in rejection of tumor cell lines expressing normal levels of MHC I molecule (Diefenbach et al., “Rae1 and H60 Ligands of the NKG2D Receptor Stimulate Tumour Immunity,” Nature 413: 165-71 (2001); Cerwenka et al., “Ectopic Expression of Retinoic Acid Early Inducible-1 Gene (RAE-1) Permits Natural Killer Cell-mediated Rejection of a MHC Class I-bearing Tumor In Vivo,” Proc Natl Acad Sci USA 98: 11521-6 (2001); Diefenbach et al., “A Novel Ligand for the NKG2D Receptor Activates NK Cells and Macrophages and Induces Tumor Immunity,” Eur J Immunol 33: 381-91 (2003)). Immune depletion and other experiments showed that the tumor rejection is due to NK cells and CD8 T cells (Diefenbach et al., “Rae1 and H60 Ligands of the NKG2D Receptor Stimulate Tumour Immunity,” Nature 413: 165-71 (2001); Cerwenka et al., “Ectopic Expression of Retinoic Acid Early Inducible-1 Gene (RAE-1) Permits Natural Killer Cell-mediated Rejection of a MHC Class I-bearing Tumor In Vivo,” Proc Natl Acad Sci USA 98: 11521-6 (2001)). NKG2D neutralization in vivo enhances host sensitivity to carcinogen-induced spontaneous tumor initiation (Smyth et al., “NKG2D Function Protects the Host from Tumor Initiation,” J Exp Med 202: 583-8 (2005)). These studies have proven the principal function of NKG2D ligand-receptor mediated NK cell immunity in tumor rejection.
In humans, the MHC class I chain-related molecule A (MICA) and MICB (generally termed as MIC) are the most investigated NKG2D ligands, which were proposed to play roles in tumor rejection (Long E O., “Tumor Cell Recognition by Natural Killer Cells,” Semin Cancer Biol 12: 57-61 (2002); Raulet D H., “Roles of the NKG2D Immunoreceptor and its Ligands,” Nat Rev Immunol 3: 781-90 (2003); Cerwenka et al., “NKG2D Ligands: Unconventional MHC Class I-like Molecules Exploited by Viruses and Cancer,” Tissue Antigens 61: 335-43 (2003)). MIC is rarely expressed by normal human tissues but induced in most human epithelial tumors (Groh et al., “Broad Tumor-associated Expression and Recognition By Tumor-derived Gamma Delta T Cells of MICA and MICB,” Proc Natl Acad Sci USA 96:6879-84 (1999); Vetter et al., “Expression of Stress-induced MHC Class I Related Chain Molecules on Human Melanoma,” J Invest Dermatol 118: 600-5 (2002); Jinushi et al., “Expression and Role of MICA and MICB in Human Hepatocellular Carcinomas and Their Regulation by Retinoic Acid,” Int J Cancer 104: 354-61 (2003); Wu et al., “Prevalent Expression of the Immunostimulatory MHC Class I Chain-related Molecule is Counteracted by Shedding in Prostate Cancer,” J Clin Invest 114: 560-8 (2004)). Expression of MIC on the tumor cell surface can markedly enhance the sensitivity of tumor cells to NK cells in vitro and has been shown to inhibit the growth of human gliomas or small lung carcinomas in experimental models (Friese et al., “MICA/NKG2D-mediated Immunogene Therapy of Experimental Gliomas,” Cancer Res 63: 8996-9006 (2003); Busche et al., “Natural Killer Cell-mediated Rejection of Experimental Human Lung Cancer by Genetic Overexpression of Major Histocompatibility Complex Class I Chain-related Gene A,” Hum Gene Ther 17: 135-46 (2006)). These studies suggest that NK cells can potentially eliminate MIC-positive tumor cells in cancer patients. However, as clinically observed, most of the human epithelial tumors are found to be MIC+ rather than MIC− (Groh et al., “Broad Tumor-associated Expression and Recognition By Tumor-derived Gamma Delta T Cells of MICA and MICB,” Proc Natl Acad Sci USA 96: 6879-84 (1999); Vetter et al., “Expression of Stress-induced MHC Class I Related Chain Molecules on Human Melanoma,” J Invest Dermatol 118: 600-5 (2002); Jinushi et al., “Expression and Role of MICA and MICB in Human Hepatocellular Carcinomas and Their Regulation by Retinoic Acid,” Int J Cancer 104:354-61 (2003); Wu et al., “Prevalent Expression of the Immunostimulatory MHC Class I Chain-related Molecule is Counteracted by Shedding in Prostate Cancer,” J Clin Invest 114: 560-8 (2004)), which suggests the functional compromise of the MIC ligand-NKG2D receptor system in cancer patients to permit the growth of MIC+ tumor cells. It has been shown that tumor-derived soluble MIC (sMIC), which occurs as a result of tumor shedding is one of the factors causing the ineffectiveness of NKG2D—mediated immunity in cancer patients (Wu et al., “Prevalent Expression of the Immunostimulatory MHC Class I Chain-related Molecule is Counteracted by Shedding in Prostate Cancer,” J Clin Invest 114: 560-8 (2004); Groh et al., “Tumour-derived Soluble MIC Ligands Impair Expression of NKG2D and T-cell Activation,” Nature 419: 734-8 (2002); Doubrovina et al., “Evasion from NK Cell Immunity by MHC Class I Chain-related Molecules Expressing Colon Adenocarcinoma,” J Immunol 171:6891-9 (2003); Raffaghello L, et al., Neoplasia 6: 558-68 (2004); Salih et al., “Down-Regulation of MICA on Human Tumors by Proteolytic Shedding,” J Immunol 169: 4098-102 (2002); Holdenrieder et al., “Soluble MICA in Malignant Diseases,” Int J Cancer 118: 684-7 (2006); Marten et al., “Soluble MIC is Elevated in the Serum of Patients with Pancreatic Carcinoma Diminishing Gamma Delta T Cell Cytotoxicity,” Int J Cancer 119: 2359-65 (2006)). In vitro studies have shown that engagement of soluble MICA to NKG2D results in marked reduction in surface NKG2D expression on NK cells and T cells (13, 16, 21). Thus, sMIC is believed to induce down-modulation of NKG2D expression on systemic and tumor infiltrated NK and T cells and thus result in functional impairment of NK and T cells in MIC+ cancer patients (Wu et al., “Prevalent Expression of the Immunostimulatory MHC Class I Chain-related Molecule is Counteracted by Shedding in Prostate Cancer,” J Clin Invest 114: 560-8 (2004); Groh et al., “Tumour-derived Soluble MIC Ligands Impair Expression of NKG2D and T-cell Activation,” Nature 419: 734-8 (2002); Doubrovina et al., “Evasion from NK Cell Immunity by MHC Class I Chain-related Molecules Expressing Colon Adenocarcinoma,” J Immunol 171:6891-9 (2003)). A reduction in the density of MIC expressed on the tumor cell surface due to MIC shedding from tumors is also one of the mechanisms for tumor evasion (Marten et al., “Soluble MIC is Elevated in the Serum of Patients with Pancreatic Carcinoma Diminishing Gamma Delta T Cell Cytotoxicity,” Int J Cancer 119:2359-65 (2006)).